varika is right the smaller the better. I used a serrated paring knife with my first sculpted cake and it was perfect. You can also freeze it again after you put your buttercream on and then do more carving just to get a bit more detail and smoothness in the buttercream itself.

I've also found that if I substitute pineapple juice instead of milk in a cake batter the cake ends up firmer (perfect for carving) and more moist.You can only just notice the difference in taste.

From what Bronwen Weber said at one of the Austin Cake Competitions she was a guest at, carving a frozen cake is a NO WAY IN HECK thing to do. It's like using a dull knife to cut something. On a frozen cake, you use more pressure w/the knife - and when the knife slips on the frozen cake - it's bodily damage time. I only carve chilled cakes, never frozen. I can't be cutting a ligament, severing a finger tip. I can always make up for a rough cut w/more buttercream and working the fondant. Can't do the same to fix a fleshwound.

How would I go about sculpting/carving a cake? Do I need to freeze it first? Please, any info!

The best tip I can give you is to use a sharp knife. The sharper the knife The less crumbs you will need to deal with. I rarely chill any of my carved caked. I just start carving with a sharp knife. Also, the size of the knife should be appropriate to the size of the cake you are carving.